1. Field of the Invention
The invention relates to a thermal head supporting device for use in a thermal printer.
2. Description of the Related Art
In the printing operation with a thermal printer, it is necessary that the thermal head is pressed against the printing paper and ink ribbon on the platen. On the other hand, when the printing paper or ink ribbon is to be inserted between the platen and the thermal head or when only the printing paper is to be fed, it is necessary that the thermal head is separated from the platen. Accordingly, there have been made various proposals for structures supporting the thermal head for movement toward and away from the platen.
For example, in the gazette of Japanese Patent Laid-open No. Hei 2-139268, there is disclosed a structure in which a frame supporting the thermal head is urged in the direction it separates from the platen, the frame is pressed to the platen by a cam, and the frame together with the thermal head is separated from the platen as the cam changes its rotational position.
Further, in the gazette of Japanese Patent Laid-open No. Hei 2-281984, there is disclosed a structure in which a head supporting plate to which the thermal head is fixed is swingably supported on a supporting shaft, the thermal head is pressed to the platen by swinging of the head supporting plate caused by urges of independent springs acting on both sides of the head supporting plate, and then both ends of the supporting shaft together with the head supporting plate are slightly vibrated by means of slits formed in opposing side plates, so that the thermal head is pressed to the platen with uniform pressure all along the width of the thermal head.
Further, in the gazette of Japanese Patent Publication No. Sho 55-39465 (Japanese Patent Laid-open No. Sho 52-119337), there is disclosed a structure in which the thermal head is fixed to a block via a coupling member, the assembly of the thermal head, the coupling member, and the block is supported with a gimbal joint, and the block is pressed by a spring so that the thermal head is pressed to the platen, or a rod provided on the block is pulled with a hook so that the thermal head is separated from the platen.
Points of problem in the related arts will be described below.
In the invention disclosed in the gazette of Japanese Patent Laid-open No. Hei 2-139268, a shaft serving as the fulcrum of rotation is passed through the frame supporting the thermal head. Therefore, in assembling or maintenance, the work to mount and demount the frame together with the thermal head is difficult. Further, while there are arranged multiple heating elements on the thermal head along the axial direction of the platen, the work to keep the line of arrangement of the heating elements parallel to the platen is impossible or very difficult.
In the invention disclosed in the gazette of Japanese Patent Laid-open No. Hei 2-281984, since the supporting shaft is passed through the side plates and the head supporting plate, the work to mount and demount the thermal head together with the head supporting plate is difficult. Further, although it is possible to make uniform the pressure of the thermal head on the platen, the adjustment to keep the line of arrangement of the heating elements on the thermal head in parallel with the platen is supposed to be very difficult.
In the invention disclosed in the gazette of Japanese Patent Publication No. Sho 55-39465, since the coupling member is supported with a gimbal joint, it is supposed that the coupling member is exchanged together with the thermal head. Further, when the gimbal joint is used, the line of arrangement of the heating elements on the thermal head cannot be adjusted to be parallel to the platen along the direction of the paper feed.
Besides, in the thermal head assemblies disclosed in the gazettes, the line of arrangement of the heating elements on the thermal head cannot be adjusted to be parallel to the platen along the direction of the paper feed, in the state of the thermal head assembly being separated from the printer as a single unit.